The only evidence for this reading comes from Latin and other ancient translations. But there are verses that are in the Sinaiticus (oldest Greek codex) that were later lost in the Byzantine Greek, but were preserved in Latin (e.g. “raise the dead” (Matthew 10:8), “the Jews” (John 3:25), “Church of God” (Acts 20:28), Doxology (Romans 16:25-27)), so it is theoretically possible for non-Greek texts to preserve an ancient Greek reading that would be lost in later Greek readings.
Moreover, even the NIV, ESV and NASB prefer several Latin Vulgate readings over readings that are preserved in the originally inspired language. The Old Testament was written in Hebrew. However, the NIV in Genesis 4:8 adds the line, "Let us go out to the field" from the Vulgate even though the line does not exist in the Hebrew. The NIV, ESV and NASB in 1 Chronicles 4:13 add "and Meonothai" from the Vulgate despite its nonexistence in the Hebrew. The NIV, ESV and NASB in 2 Chronicles 15:8 add "Azariah the son of" from the Vulgate despite its nonexistence in the Hebrew. Anybody who uses the NIV, ESV or NASB has no right to fault the KJV for including a line from the Vulgate.
Even if one were to doubt the KJV reading of Acts 9:5-6, he has no reason to doubt the historical fact stated therein. This exact conversation between Christ and Paul did occur, according to Acts 22 and 26. So even if a reader supposes that including this passage in Acts 9:6 is an error, the reader has no reason to doubt the truth of the passage. This is not a case where the stated historical fact is not supported by any original language text. For example, the NIV has Cain saying to Abel, "Let's go out to the field" in Genesis 4:8 based on non-Hebrew texts (i.e. Vulgate, Septuagint). The Hebrew does not have this reading in Genesis 4:8 and no other place in the Bible mentions this statement by Cain to Abel. KJV's Acts 9:6 is not such a case where a debatable historical fact is included. In conclusion: At most, one could accept the KJV reading as original presuming that non-Greek texts preserved the reading correctly against all Greek texts that became corrupt in this place at a very early stage. But at the very least, even if one were to think that the KJV incorrectly inserts this passage in Acts 9:6, he has no reason to doubt the content. We can read Acts 9:6 and believe its content with certainty.
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Also read: The Greek Text (Textus Receptus) of the King James Version is Reliable